Field
The present invention relates to a TO-can type laser device, and more particularly, to a TO-can type laser device having an optical isolator function that prevents light emitted from a laser diode chip from being again returned to the laser diode chip therein.
Description of the Related Art
Communication services having very large communication capacity including a video service of a smartphone, or the like have recently been released. Accordingly, a need to significantly increase conventional communication capacity is emerged, and as a method of increasing communication capacity using an optical fiber which is conventionally installed, a communication scheme of dense wavelength division multiplexing (DWDM) has been adopted. The DWDM refers to a method of simultaneously transmitting lights at various wavelengths through one optical fiber using a phenomenon that does not cause interference between optical signals even if the optical signals of several wavelengths are simultaneously transmitted through one optical fiber due to a fact that laser lights having different wavelengths do not interfere with each other.
Recently, a standard called Next Generation—Passive Optical Network version 2 (NG-PON2) has globally been negotiated, and in the NG-PON2 standard, as a standard of optical communication module to be installed in a subscriber, laser of a frequency interval of 100 GHz has been adopted. An optical module for a subscriber of the NG-ON2 has a transceiver module called small form factor pluggable (SFP) as a basic standard. Since a volume of an SFP module package is small, a size of a laser module should be miniaturized.
In addition, in digital optical communication in which a signal of “1” and a signal of “0” are distinguished depending on intensity of light emitted from the laser diode chip, the signal of “1” and the signal of “0” are adjusted by the intensity of light emitted from a semiconductor laser diode chip. The intensity of light emitted from the semiconductor laser diode chip is varied depending on intensity of a current injected into the semiconductor laser diode chip, and a chirp phenomenon occurs in which a wavelength of chip laser light is varied depending on the intensity of current the current injected into the semiconductor laser diode chip. In a high speed operation of 10 Gbps, the chirp phenomenon occurs in which spectrum of the laser light emitted from the laser diode chip is widened, and the above-mentioned chirp phenomenon has a disadvantage that it is combined with dispersion characteristics of the optical fiber thereby shorten a transmission distance of an optical signal.
The above-mentioned chirp phenomenon is combined with dispersion characteristics of the optical fiber to thereby make a long distance transmission of a high speed optical signal difficult. In order to decrease the chirp phenomenon, a method of decreasing a modulation width of the current injected into the semiconductor laser diode chip generating the signal of “1” and the signal of “0” may be used. However, in the case of using this method, since a difference between intensities of the signal of “1” and the signal of “0” is decreased, a problem that it is difficult to distinguish the signals occurs,
In order to solve the above-mentioned problem, a method has been used in which light corresponding to a wavelength of the signal of “0” of the laser diode chip is optically decreased to electrically drive the laser diode chip so that the modulation width of the current is not large and to then optically emphasize the difference between the intensities of the signal of “1” and the signal of “0”.
FIG. 1 shows a structure of a chirp managed laser described in U.S. Pat. No. 8,199,785. The chirp managed laser uses a method of selectively decreasing light corresponding to the signal of “0” using an optical filter to increase a modulation width of a final output signal. Here, since the laser light oscillated from the semiconductor laser diode chip has a change of a wavelength of about 0.1 nm/°C. depending on a temperature, and the optical filter should selectively decrease only the signal of “0”, there is a problem that wavelengths of light emitted from the optical filter and the semiconductor laser diode chip need to be precisely matched to each other. For this reason, a wavelength stabilizer is essential in the chirp managed laser, and reference numerals 308, 310, and 312 of FIG. 1 denote structures for wavelength stabilization described above. FIGS. 2 show a view (FIG. 2A) showing a ratio of intensities of signal wavelengths of the signal of “1” and the signal of “0” in a case in which the optical filter is not used, and a view (FIG. 2B) showing a ratio of intensities of signal wavelengths in a case in which a difference between the intensities of the signal of “1” and the signal of “0” is expanded using the optical filter that relatively well passes the intensity of the signal of “1” and relatively interrupts the intensity of the signal of “0”.
In order to use the wavelength stabilizer of FIG. 1, a portion of the light emitted DFB-LD 302 (FIG. 1) needs to be reflected from OSR 304 (FIG. 1) and to be again moved in a direction of the DFB-LD 302 (FIG. 1). In particular, in a case in which the optical filter has characteristics of etalon, since characteristics of the etalon optical filter become good when the etalon optical filter is disposed so that incident light is incident to be almost vertical to an incident surface of the etalon optical filter, most of the etalon optical filters are designed so as to correspond to light entering the incident surface of the etalon optical filter at a narrow angle of about +/−3 degrees of a vertical angle. Light reflected vertically from the etalon filter may be fed back to the laser diode chip to thereby disturb an operation of the laser diode chip. However, since the DFB-LD 302 (FIG. 1) has characteristics which very sensitively change for light entered from the outside, a method is required that allows light moved from the OSR 304 (FIG. 1) to the DFB-LD 302 (FIG 1.) to be moved in PD 312 (FIG. 1), and not to be moved in DFB-LD 302 (FIG. 2). According to the relate art of FIG. 1, an optical isolator 306 (FIG. 1) that passes light only in one direction and does not pass the light in an opposite direction is disposed.
In FIG. 1, two optical isolators are shown, in which the optical isolator 306 (FIG. 1) has a function of blocking the light emitted from the laser diode chip and reflected from a wavelength selective filter 304 (FIG. 4) so as not to be fed back to the laser diode chip, and another optical isolator 324 (FIG. 1) is an optical isolator for blocking light injected into an optical element of FIG. 1 through an optical fiber from another optical element.
Since the optical isolator includes two polarizers, a Faraday rotator disposed between the two polarizers, and a permanent magnet applying a magnetic field to the Faraday rotator, the optical isolator has a problem that it is complex and expensive.
In FIG. 1, the laser light emitted from the semiconductor laser diode chip 302 (FIG. 1) goes straight to be incident on an optical fiber 318 (FIG. 1), and this type of configuration is most preferably implemented by FIG. 3 of U.S. Pat No. 8,199,785 according to the related art, which is called a mini-flat type. However, since an optical element package housing of the mini-flat type is expensive, a method of implementing the wavelength stabilizer using a TO-can type package which is inexpensive is shown in US20150200730 of FIG. 4. As the TO-can type, a small product typically having a diameter within 6 mm is mainly applied to optical communication. The reason that a size of 6 mm is important in the TO-can type package having the diameter of 6 mm or less is because a height of a package of an SFP type transceiver, which is the most important standard of a current optical transceiver for communication is about 8 mm, and only the TO-can type package having the maximum diameter of 6 mm is allowable to be mounted within the above-mentioned package.
In US20150200730 of FIG. 4, when a filter 400 is mounted within the above-mentioned. package, used as the optical filter, the chirp managed laser having the wavelength stabilizer embedded therein may be implemented in the TO-can type. However, since the case of FIG. 4 does not have a device that prevents the feedback of the light such as the optical isolator 306 (FIG. 1) shown in U.S. Pat. No. 8,199,735 of FIG. 1, a portion of light emitted from the laser diode chip 100 (FIG. 4), entering an optical filter 400 (FIG. 4), reflected from the optical filter 400 (FIG. 4) and then again moved to a 45 degree partial reflection mirror 300 (FIG. 4) is reflected from the partial reflection mirror 300 (FIG. 4) to enter the semiconductor laser diode chip 100 (FIG. 4), thereby making it possible to disturb an operation of the semiconductor laser diode chip 100 (FIG. 4). In the case of FIG. 4, in order to obtain strong light intensity, the 45 degree partial reflection mirror 300 (FIG. 4) may reflect most of the incident light. Therefore, since most of the light reflected From the optical filter 400 (FIG. 4) to arrive at the 45 degree partial reflection mirror 300 (FIG. 4) is reflected to the laser diode chip 100 (FIG. 4), the operation of the laser diode chip 100 (FIG. 4) is more easily disturbed.
In the case of FIG. 4, as a type in which the optical isolator may be disposed, a method of disposing the optical isolator at a position of an optical isolator (A) of FIG. 5 is possible. However, it is very difficult to apply this method to a limited size of the diameter of substantially 6 mm.
FIG. 6 shows a TO-can internal structure of a structure of FIG. 4 manufactured in the TO-can type having a diameter of substantially 6 mm. In a case in which an outer cap portion of a package in a TO-can type package having the diameter of 6 mm is considered, a space in which a past may be disposed in the TO-can type package is limited to 4 mm or less. In FIG. 6, a size of a collimating lens is 0.7 mm, and in a case in which the size of the lens is decreased to 0.7 mm or less, since a size of a collimated beam becomes too small, characteristics of transmission/interruption by interference in the etalon filter do not exhibit well. In the TO-can type, since the collimated light is preferably emitted in a concentric axis of the TO-can type package, the 45 degree partial reflection mirror is preferably disposed on the concentric axis of the TO-can type package. As shown in FIG. 6, since the TO-can type package having the diameter of 6 mm has a very small size, a problem occurs that a space in which an optical isolator typically having a size of 1 mm or more is to be mounted is insufficient. Therefore, it is substantially impossible to dispose the optical isolator at the position (A) of FIG. 5. In the TO-can type package, the optical isolator such as the optical isolator (B) of FIG. 5 may be disposed without limiting the size of the package in a vertical upward direction of the 45 degree partial reflection mirror to thereby prevent the light reflected from the optical filter 400 (FIG. 5) from being fed back to the laser diode chip 100 (FIG. 5). However, in this case, since light to be incident to a photo diode 500 (FIG. 5) is also interrupted by the optical isolator (B), by which the wavelength stabilizer is not used, the function of the chirp managed laser may not be implemented.